Diacyl peroxides are among the commonly used initiators in the commercial production of polyolefins, particularly fluoroolefins, such as tetrafluoroethylene. They may be represented as R—(C═O)—O—O—(C═O)—R. The peroxide decomposes to give R., known as a free radical, which reacts with olefin monomer to begin the polymerization cycle. Taking tetrafluoroethylene as an example:R—(C═O)—O—O—(C═O)—R→2R—(C═O)—O.→2R.+2CO2R.+CF2═CF2→R—CF2—CF2. R—CF2—CF2.+CF2═CF2→R—CF2—CF2—CF2—CF2.The R group arising from the initiator is called an “endgroup” of the polymer.
The classical synthesis of diacyl peroxides is an aqueous synthesis. An alkaline aqueous solution of hydrogen peroxide is contacted with a water-immiscible solution of acid halide. Examples are found in S. R. Sandler and W. Karo, (1974) Polymer Synthesis, Vol. 1, Academic Press, Inc., Orlando Fla., p. 451 and U.S. Pat. No. 5,021,516. This is a reaction of two liquid phases, an aqueous phase and a nonaqueous phase. Equation (1) shows the reaction:2R—(C═O)X+H2O2+2NaOH→R—(C═O)—O—O—(C═O)—R+2NaX+2H2O  (1)From the stoichiometry of (1) it is clear that one mole of hydrogen peroxide reacts with two moles of acyl halide to yield one mole of diacyl peroxide. The diacyl peroxide as it forms is taken up in the water immiscible phase. By this means, exposure of the acyl halide and the diacyl peroxide to the alkaline aqueous phase is minimized, which is desirable because water hydrolyzes both the organic acyl halide starting material and the diacyl peroxide product. Hydrolysis decreases yield and introduces byproducts such as acids and peracids, which are impurities. At the end of the reaction, the water-immiscible solvent with the diacyl peroxide dissolved in it is separated and dried, and purified as necessary.
The use of sodium percarbonate and sodium perborate for aqueous and nonaqueous functional group oxidation in organic synthesis has been reported (A. McKillop and W. R. Sanderson, Tetrahedron, vol. 51, no. 22, pp. 6145–6166, 1995; J. Muzart, Synthesis pp. 1325–1346, November 1995). Organic oxidation reactions using urea/hydrogen peroxide adduct in protic and aprotic solvents is reported by M. S. Cooper, et al. Synlett, pp. 533–535, September 1990. Organic acids, acid anhydrides, and acyl halides are reagents and solvents in these reactions and peracids are proposed as intermediates in the oxidations. Formation of diacyl peroxide as an undesirable byproduct of the reaction is discussed.
Japanese Patent 61152653 discloses the preparation of diacyl peroxides by the mixing of acyl halides with sodium peroxide (Na2O2) in halogenated solvent, followed by addition of water. Sodium peroxide must be handled carefully: it can react violently or explosively with organic materials, is hygroscopic, and absorbs carbon dioxide from air to form compounds that can ignite if subjected to pressure or friction. Furthermore, sodium peroxide, being strongly basic, cannot be used with compounds that are sensitive to bases.
A safe, economical, synthesis of diacyl peroxides in aprotic solvent using stable, easy to handle oxidizing agents, and proceeding in high yield is needed. Looking to the future, the need is greatest for a process that can be run in a nonflammable solvents such as fluorocarbons, chlorofluorocarbons, or certain hydrohalocarbons.